ETT size and depth (pediatric)

Uncuffed internal diameter (mm) = age in years ÷ 4 + 4 (Cole). Cuffed internal diameter (mm) = age in years ÷ 4 + 3.5, pinned to the APLS/Motoyama/Duracher constant; the classic Khine +3.0 gives a tube 0.5 mm smaller and is still widely taught. Oral depth at the lips (cm) = age in years ÷ 2 + 12. Each raw diameter is also snapped to the nearest manufactured 0.5 mm size, with exact half-steps resolved DOWN, both because that reproduces the conventional taught sizes (1 y cuffed 3.5, 3 y 4.0, 5 y 4.5) and because the errors are asymmetric: a tube 0.5 mm small is exchanged or tolerated, one 0.5 mm large is the mechanism of subglottic injury.

Scope is age 1 to 12 years, with the lower bound enforced: below 1 year these formulas are invalid and are refused, so use weight- and gestational-age-based neonatal sizing instead. Above about 12 years, use adult sizing. The ID × 3 depth rule is deliberately not emitted: it diverges from age ÷ 2 + 12 by about 2 cm at 1 year and about 3 cm at 12, so printing both would put two oral depths for one child on one screen. Keep tubes 0.5 mm larger and smaller at hand. The formulas mis-size a meaningful minority of children (Cole over-sizes the youngest in range), so confirm by air-leak test (about 20 to 30 cmH₂O uncuffed), auscultation, capnography, chest rise, and imaging. [NEEDS SOURCE]: the nasal-route depth offset (about +2 to 3 cm) and the fixed infant depth steps are unverified and not implemented.